One of the great advantages of MJML is that it's component-based. Components abstract complex patterns and can easily be reused. In addition to the standard library of components, it is also possible to create your own components!
We have published a step-by-step guide here that explains how to create a custom components with MJML 4. It will introduce to you the boilerplate repo hosted on Github, which provides a fast way of getting started developing your own components.